Lightning-arrester.



No. 793,445. v PATENT'ED JUNE 27,1905.

' T. J. JOHNSTON.

LIGHTNING A'RRESTER.

APPLIOATION rmm NOV. 8, 1901.

UNITED STATES THOMAS J. JOHNSTON, OF BROOKLYN, NEW YORK, ASSIGNOR TOGENERAL Patented June 27, 1905.

PATENT OFFICE.

ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

vLlC-EHTNINGi-ARRESTEFL SPECIFICATION forming part of Letters Patent No.793,445,'dated June 27, 1905.

Application filed November 8, 1901. Serial No 81,637.

To all whom, it may concern:

' Be it known that I, THOMAS J. JOHNSTON, of Brooklyn, in the county ofKings and State of New York, have made certain new and usefulImprovements in Lightning-Arresters, of which the following is aspecification.

My present invention relates to lightningarresters, and has for itsobject to improve the protection which these devices afford toelectrical machiner; generating current at such potential as is liableto cause an arc to follow the action of the lightning-arrester when itrelieves the line of static charge.

Under some conditions, as when the discharge is of arapidly-oscillatingcharacter or when it consists of a single impulsive dischargepractically instantaneous and often of high potential, I have found itof advantage to associate with the spark-gap a device havingelectrostatic capacity. In many instances it will be found sufficient toprovide upon one side of the spark-gap a plate of some size connectedwith the ground-line or forming a part of it. The effect of this is toweaken the dielectric at the spark-gap, or rather to put it understrain, so that it tends to break down readily, or, in common parlance,attracts the discharge. This, however, is not the only function ofelectrostatic capacity when associated with-the spark-gap of alightning-arrester where the latter is used on a dynamic circuit. Undersuch conditions, as is well known, the machine-current follows thelightning discharge across the sparkgap, often giving rise to disastrousarcs which may, where considerable energy is involved, cause thedestruction of the apparatus. In fact,

this is the main reason for lightning protection, since under mostconditions the mere diversion of the static discharge is effected withcomparative ease, at least except where the direct stroke of lightningis concerned. Under these conditions it is of value to employelectrostatic capacity at the spark-gap, though usually in a differentform from the simple one above outlined. Where reasonably highpotentials are used, it is desirable to employ a condenser in shunt tothe sparkgap, since the machine-current following the discharge tends tovary in such way as to charge the condenser, and by thus deflectingcurrent from the are between the spark-gaps renders the are unstable andto a certain extent self-interrupting. Thus the condenser permits theuse of a smaller resistance in the ground-circuit, it being desirable,as is well understood, to keep this resistance as small as is consistentwith a proper limiting of the afterflow of machine-current.

An additional advantage of the arrangement proposed is that lightningprotection is usually comparative that is to say, it being generallyimpossible to obtain an electric circuit with no inductance the endeavorhas been to increase the inductance of the machinerycircuit or that partwhich requires protection and to diminish as far as possible theinductance of the path which it is desired that a static discharge shalltake. This has in many cases been accomplished by interpolating in theprotected circuit an artificial inductance, generally in the form of anopen coil especially designed for this class of reactance. This hasbecome a recognized and valuable adjunct to a lightning-arrester. Itsuse in alternating-current circuits, however, is objectionable inmanycases, since it constantly consumes energy on all circuits of commercialfrequency. Nevertheless, it has been often applied even onalternating-current circuits.

By my invention it is manifest that the amount of artificial inductanceneeded to protect any circuit may be reduced to the same extent that theinvention reduces the inductance of the ground-circuit, by which I amable to obtain both the advantages pointed out-namely, the reduction ofthe ohmic and inductive resistance of the ground-circuit and thereduction of the artificial inductance necessary in the protectedcircuit.

The accompanying drawings show in coninvention to which I have referred.

Figure 1 is a diagram of the grounded circuit having the first form ofthe invention apventional diagram the two applications of the pliedthereto. Fig. 2 is a diagram representing a metallic alternating circuitwith the second form of the invention.

In Fig. 1, A is a generator of the compound or railway type having oneterminal grounded and the other connected to the line B. O is anartificial inductance or kicking coil, as it is commonly called, inseries in the line B. I) is the spark-gap of the lightning-arrester, oneside of which is grounded, and in the ground-circuit is a non-inductiveresistance E. This resistance is of any form which will not increase theinductance of the grounded circuit, a common type being a carbon rod theresistance of which is proportioned to the line-voltage in well-knownways. F is a blowout magnet which is connected in shunt to a part of theresistance and acts upon the are, following the lightning dischargeacross the terminals I), the coil of the magnet being energized by thedrop in that part of the resistance spanned by its terminals.Electrically connected to a terminal of the spark-gap is a device havingelectrostatic capacity, such, for example, as a plate of good conductingmetal, (indicated diagrammatically at Gr.) This may be of any convenientform or size, but should preferably have considerable surface. It iswell understood that it is not necessary that it should be a single flatplate, but may be folded or rolled, provided the surfaces of the platedo not come into electrical contact.

In Fig. 2 the same letters refer in general to the same parts, but herea different form of lightning-arrester is employed. The system beingalternating is not grounded, and lightning-arresters now in common use,consisting of a number of substantial cylinders, of a good conductingmetal, insulated from each ,other and having their surfaces brought intoproximity without contact, so as to form a series of spark-gapsextending from one line to the other, are employed. The number of thegaps will depend upon the voltage of the line, as is well understood,and commonly the potential, as is well understood. The calculi);

tion of the electrical constants being within the knowledge of everyengineer it is unnecessary to further refer to them.

Having thus described my invention, what I claim, and desire to protectby Letters Patent of the United States, is

1. The combination of a circuit to be protected, a series of spark-gapterminals connected directly to said circuit, a grounded connection tosaid spark-gap terminals containing a non-inductive resistance, and adevice possessing capacity connected to said sparkgap terminals.

2. In a lightning-arrester, the combination of a plurality of spark-gapterminals, a direct connection between one of said terminals and thecircuit to be protected, a ground connection from another of saidterminals, a noninductive resistance in said ground connection, and adevice possessing electrostatic capacity associated with the spark-gapterminals.

3. In a lightning-arrester, the combination with a protected circuitcontaininginductance. of a grounded circuit having a spark gap or gapsand a non-inductive resistance in series, and a device havingelectrostatic capacity associated with said grounded circuit.

In witness whereof I have hereunto signed my name, in the presence oftwo witnesses, this 6th day of November, 1901.

THOMAS J. JOHNSTON.

l/Vitnesses:

ERMINA E. WALKER, DANIEL J. MONAMARA.

